1. Field of the Invention
The present invention relates to a digital color image processor.
2. Description of Prior Art
In a digital color image processor, black characters in a document are usually discriminated to improve reproducibility of black characters. Characters (edges) are discriminated from achromatic image portions in a document by detecting an edge portion and an achromatic portion in the document. Various techniques are proposed for discriminating an achromatic image, and they discriminate an achromatic portion according to color data, that is, red (R), green (G) and blue (B) data. For example, in a technique, a difference of the maximum from the minimum of R, G and B color data, is used as chroma data. A portion having a larger difference is decided to be a color image, while a portion having a lower difference is decided to be an achromatic image.
It is also known to discriminate a dot area in a document and prevents Moire phenomenon by using the discrimination. In the discrimination, isolated dots isolated are detected from the gradation distribution of color data, and a dot area is detected according to the number of the dots in a unit pixel matrix.
On the other hand, in such an image processor, image forming mode such as the magnification or the density of an output image is changed automatically or manually by a user. The magnification can be changed by controlling resolution of color data. The density can be changed by controlling gradation characteristic such as background level.
When the gradation characteristic is changed for density control, the chroma data and the edge level are affected. For example, when the image reading conditions are set so as to remove the background of a document, it becomes difficult to decide an edge of a character having a low density. Further, when an image is reduced by changing magnification, a character having many lines therein is erroneously decided as dots. Thus, the quality of image reproduction is deteriorated.